1. Field of the Invention
The invention pertains to the field of sprockets for roller or bush chains. More particularly, the invention pertains to a sprocket that reduces the noise generated when the roller or bush chain and the sprocket mesh.
2. Description of Related Art
Conventional chain sprocket interfaces seat the roller or bush in the root of the sprocket tooth as if the sprocket is held stationary and a taut chain is wrapped around it. A roller swinging-on an arc the length of a chain pitch around the center of the previous roller, located on the pitch circle of the sprocket, contacts the root of the sprocket tooth when reaching the pitch circle of the sprocket.
FIG. 11 shows a chain C having rollers R engaging sprockets S1 and S2. Noise generated by a roller chain meshing with a sprocket is a long recognized problem. One of the most significant sources of noise in a typical roller chain drive results from the repeated impacts between the rollers and the sprockets as the rollers seat between the teeth of the sprocket. As shown in FIG. 12, a roller 10 engaging a sprocket S swings on an arc 20 having a radius equal to the chain pitch P around the center of a seated adjacent roller 30. The roller 10 swings along the arc to make a substantially radial impact with the sprocket tooth root 40.
The prior art discloses attempts to reduce the noise created by rollers impacting a sprocket. Some sprocket designs have attempted to decrease the impact noise of roller chain drives by altering the sprocket tooth profile. One attempt to reduce roller chain and sprocket noise using a modified sprocket tooth profile is described by U.S. Pat. No. 5,921,877. A sprocket tooth profile having three sections is described as making the roller of a chain come into engagement with the teeth of the sprocket without generating a large noise. However, this design is limited to a unidirectional roller chain drive because of the asymmetry of the tooth profile as generally shown by FIG. 13.
Other designs also utilized an asymmetrical tooth design in an effort to reduce the noise generated by the impact of the rollers as they strike the sprocket. U.S. Pat. Nos. 5,921,878 and 5,993,344 disclose an asymmetrical tooth profile that incorporates a flank flat that is tangential to an engaging flank radius and a first root radius, and a pitch mismatch, where the sprocket chordal pitch is less than the chain chordal pitch. This arrangement is described as facilitating a staged roller tooth contact with the sprocket. Again, the asymmetry of the tooth profile mandates a unidirectional roller chain drive system.
Other asymmetrical tooth designs provide a tooth space clearance between the roller and a portion of the tooth root in an attempt to reduce the noise generated by the vibrational contact of unseated rollers with the sprocket teeth as the rollers wear. FIG. 14 illustrates one such design. Instead of impacting the bottom of the tooth root, the roller makes radial contact with the flanks of adjacent teeth at locations A1 and A2 while bridging over the bottom of the tooth root. This sprocket is also designed for a unidirectional roller chain drive system. Similarly, U.S. Pat. No. 5,397,278 simply reduces the gap between adjacent sprocket teeth, such that the roller makes simultaneous radial contact with the flanks of adjacent teeth. However, this design may be used in a multi-directional roller chain drive system.
U.S. Pat. No. 6,736,744 discloses a sprocket that substantially avoids radial impact of chain rollers with the sprocket. The tooth root radius of the sprocket is increased while the sprocket pitch circle diameter is maintained. Thus, a clearance between the roller and the tooth root is created, which allows rollers of a chain of like pitch to impact the sprocket teeth in a substantially tangential fashion on their flanks, rather than substantially radially on the tooth root. Backlash is not minimized with this design.
Similarly, U.S. Publication No. 2003/0186766 discloses an engaging flank profile with seating, impact and distal portions, all with arcs with well-defined radii and arc centers. The profile of the disengaging flank is determined in accordance with a modified form of the JIS standard, in which the sprocket pitch is smaller than the roller chain pitch. The impact point is located at the tangent point to permit a gradual change in the roller velocity. Backlash is not minimized with this design.
Therefore, there is a need in the art for a multi-directional sprocket that substantially minimizes the noise generated by the substantially radial impacts between the rollers of a roller chain and a sprocket, while minimizing backlash.